In circulating fluidized bed catalytic cracking units, spent catalyst is reactivated in regenerator by burning the coke deposited using air as the oxidant. The flue gas generated during the regeneration step is released to the atmosphere after reducing the pressure and recovering the heat. The regenerator is typically operated at a pressure range of 1.5 to 3 kg/cm2 (g) and in the temperature range of 650 to 750° C. The regenerator pressure is controlled by a double disc slide valve (DDSV). Typically, an orifice chamber is provided at the downstream of DDSV for progressively & gradually reducing the pressure of flue gas. Each orifice chamber is equipped with one or more orifice plates, and typically it has a minimum of two and a maximum of seven orifice plates in order to progressively take small pressure drops across the orifice plates instead of single step pressure reduction. DDSV opening varies as directly proportional to the square of the velocity of flue gas. The typical orifice velocities across the DDSV and orifice chamber are maintained about 250 m/s and 150 m/s respectively. Theoretically, it is possible to operate DDSV and orifice chamber at much higher velocities but with increased noise and erosion of orifices.
A gap of 60 cm between successive orifice plates is maintained for periodic inspection and maintenance in orifice chamber. Further, orifice chamber is equipped with multiple man-ways or manholes for access to each orifice plates for inspection and repair or replacement. In such locations, 100 to 120 cm gap is provided between successive orifice plates. The diameter of the orifice chamber ranges typically from 75 to 350 cm and the ratio between diameters of orifice chamber to the diameter of inlet flue gas line is typically 5 to 10. Due to the difference in diameter, a sudden flow expansion takes place at the entrance of the orifice chamber. The flue gas leaving the flue gas line forms a jet in the initial portion of orifice chamber and subsequently disperses across the orifice chamber and a fully-developed flow region is achieved. Due to this, the first orifice plate is provided much below the exit of the flue gas pipe, typically 250 to 350 cm to ensure that the flue gas flow is fully developed before it passes through the orifices of the plate.
During the revamp of the unit, the pressure of reactor and regenerator goes up by 1 to 2 kg/cm2 due to higher feed input and higher air rate. In such cases, 1 or 2 additional orifice plates are to be installed to reduce the pressure of the flue gas. The flowing medium is typically flue gas from a catalytic cracking unit going to the stack outlet of a refinery. The flue gas is a result of the combustion process in a regenerator of a catalytic cracking unit.
FIG. 1 illustrates a longitudinal view of a section of a prior art assembly of orifice chamber 10 conventionally in-use which requires a flow length approximately 10 times the diameter ‘D’ of an inlet flue gas line 12 for achieving a fully developed flow in the initial portion 11 of the orifice chamber defined in a region prior to the location of the first orifice plate 16. The diameter of the inlet flue gas line 12 is represented as ‘D’ in FIG. 1. Further, the prior art of FIG. 1 illustrates a first office plate 16, and additional orifice plates, 18, 20 and 22 is located within the orifice chamber 10. The location of the first orifice plate 16 from the exit of the inlet pipe 12 may be approximated as 10D, where 10D defines a length of the fully developed flow in the initial portion 11.
Accommodating the additional orifice plates in existing orifice chamber is difficult as space between the orifice plates is limited for inspection & maintenance. Installing auxiliary orifice chamber with 1 or more orifice plates requires a separate permanent civil structure, which demands more no of shutdown days. If additional grids are not installed, the load in terms of pressure drop increase on DDSV, which causes erosion on DDSV discs.
It would be highly desirable to provide an improvement in the present assemblies of such orifice chambers which would enable to accommodate additional orifice plates without scarifying the clearance gap between the orifice plates.